1. Field of the Invention
This invention relates to a novel contact lens of the type applied to the human eye to correct vision deficiencies
2. Description of the Prior Art
Although known and used for years, contact lenses have not been accepted fully by most persons having vision difficulties. Contact lenses are generally of two types, scleral and corneal. Those of the scleral type cover a substantial area of the eye during use. This tends to seal off circulation of tears and the atmosphere, thereby causing partial asphyxiation which affects the metabolism and vision of the eye. The more recent corneal lenses, which because of their relatively smaller size and lighter weight, may cause less irritation, have, therefore, achieved somewhat greater acceptance. However, the "break-in" period necessary to accustom the eye of the wearer to support a corneal lens, as well as a scleral lens, usually extends over a considerable time, and some persons have found contact lenses in any form intolerable because of the eye irritation and even corneal changes that occur during and after extended periods of wear.
Contact lenses of both the corneal and scleral types have been commonly constructed from poly (methyl methacrylate) (PMMA) which has a Knoop hardness of about 20. This material has the advantage of being both lightweight and machinable, but when a PMMA corneal contact lens is introduced for the first time into the eye of a patient, the sensation is that of a foreign body being introduced. A fraction of those who try PMMA lenses become adapted to them after the break-in period so that they can successfully wear them with little or no discomfort. A larger fraction of those who attempt to wear them never become wearers because they cannot adapt to the lenses. Even those individuals who adapt successfully to PMMA lenses suffer an awareness of the lens in the eye. This is partly because of the stiffness of the lens and partly because the eye becomes deprived of oxygen due to the low permeability of PMMA to oxygen.
Many attempts have been made to design scleral and corneal contact lenses for comfort and for proper fluid circulation. U.S. Pat. No. 3,542,461 discloses the advantage of constructing contact lenses from a material having a refractive index near the refractive index of tears, i.e., about 1.336. U.S. Pat. No. 3,228,741 discloses the advantage of constructing lenses from a material having high O.sub.2 and CO.sub.2 permeability to reduce eye irritation caused by asphyxiation and suggests the use of a transparent silicone rubber to achieve this high permeability. U.S. Pat. No. 3,551,035 suggests a more rigid material, poly(4-methyl pentene-1) for the same purpose. Finally U.S. Pat. No. 3,220,960 discloses the comfort advantage of lenses which are made from soft hydrogel materials, but these materials generally suffer from a lack of dimensional stability, which causes the power of the lens to change during wear. They are also prone to bacterial growth, because of the high water content, and also appear to draw water from the eye.
An ideal contact lense, by definition, lens, a contact lens having an index of refraction close to that of tears, a reasonably high permeability to oxygen and carbon dioxide and a low water content. The lens should also be as soft and flexible as possible, yet hard enough to be machined and polished. The alternative process of molding the lens is feasible, but expensive since a large number of molds would be needed to satisfy the various optical needs of prospective wearers. The advantages of each of the various properties has been recognized, but no one has yet been able to produce a contact lens which has an optimum combination of these properties. The proponent of each property merely seeks a material having the particular advantage in which he is interested and generally accepts what he can get relative to the other properties. I have invented a new contact lens which has a combination of properties that are so close to the ideal combination of properties that the lens can indeed be considered an ideal contact lens.